The present invention relates to a subsea flexible pipe for conveying hydrocarbons in deep water.
Flexible pipes for conveying hydrocarbons are already well known, and they generally comprise, from the interior to the exterior of the pipe, a metal carcass, a polymer internal sealing sheath, a pressure armor layer, tensile pressure armor layers, and a polymer external sheath to protect the overall pipe and in particular to prevent seawater from penetrating its thickness. The metal carcass and the pressure armor layer comprise longitudinal elements wound in a short pitch, which enable the pipe to withstand radial forces, while the tensile pressure armor layers comprise metal yarns wound in a long pitch to absorb the axial forces. The type, number, dimensions and organization of the layers constituting the flexible pipes are essentially related to their conditions of use and installation. In the present application, the short pitch winding concept designates any helical winding at a helix angle close to 90°, typically between 75° and 90°. The long pitch winding concept concerns helix angles lower than 55°, typically between 25° and 55° for the armor layers.
These flexible pipes are suitable for conveying hydrocarbons, in particular on the seabed and at great depths. More precisely, they are referred to as unbonded, and are thus described in the standards published by the American Petroleum Institute (API), API 17J and API RP 17B.
When the flexible pipe, regardless of its type, is subjected to an external pressure that is higher than the internal pressure, an axial compression may occur, known to a person skilled in the art as the reverse end cap effect. The reverse end cap effect tends to compress the flexible pipe axially, to shorten it, and to increase its diameter, thereby tending to cause a swelling of the tensile pressure armor layers. In the case in which the external sheath of the pipe is sealed, the hydrostatic pressure prevailing outside the pipe effectively opposes the swelling of the tensile pressure armor layers. On the contrary, if the external sheath is no longer sealed, for example due to an accidental tear, the hydrostatic pressure no longer opposes the swelling of the tensile pressure armor layers. In the absence of an additional means for limiting this swelling, the yarns constituting the tensile pressure armor layers are then liable to buckle radially, which may cause an irreversible local deformation of said armor layers into a “birdcage” shape, and thus lead to the failure of the pipe.
One known solution for reducing this risk of radial buckling into a “birdcage” is the short pitch winding of reinforced strips of aramid fibers, and more precisely of poly(paraphenylene terephthalamide) (PPTA) homopolymer fibers, around the tensile pressure armor layers. Such strips have a high mechanical tensile strength along their longitudinal axis, thereby limiting the swelling of the tensile pressure armor layers. They also have great bending flexibility, which facilitates the operations of handling and winding around the armor layers. Finally, with equivalent mechanical properties, they are much lighter than metal strips, thereby reducing the weight of the flexible pipe. Reference can be made in particular to document FR 2 837 899 in which such a pipe is disclosed.
These reinforcing strips are in the form of bundles of rovings comprising PPTA homopolymer fibers directed parallel to the longitudinal axis of the strip. These longitudinal rovings can be joined together into a relatively flat bundle having a substantially rectangular cross section like that of a strip or tape. It is also possible to use a reinforcing strip comprising a substantially rectangular central section and two longitudinal edges thinner than the central section, as described in document EP 1419338. The means for joining and restraining these rovings generally comprise transverse elements which are shaped so as to surround and clamp said rovings together into a relatively flat bundle. In common configurations, these transverse elements are treated as weft yarns, the rovings forming the warp, and the strip can then be considered as a woven material. Various embodiments of these reinforcing strips are described in documents WO 97/12753 and WO 9713091.
However, despite this, it has been found that in extreme service conditions, these reinforcing strips can deteriorate. These extreme conditions are chiefly encountered when the flexible pipe is submerged at great depth, typically at more than 2000 m, and simultaneously subjected to severe dynamic bending loads, thereby causing fatigue of the reinforcing strips.